Conventional Narrow Band, On/Off or frequency shift keying Power Line Carrier equipment is used to assist sub-station and generating station fault protection relays with isolating a power line fault (overload current) to a segment of the power grid where the fault takes place. Circuit breakers on both sides of the fault are tripped to prevent the overload from taking down more of the grid.
This type of Power Line Carrier is commonly referred to as PLC. It is an RF transmitter and receiver that operate over AC transmission lines with voltage in the tens to hundreds of KV, and line distance of a few miles to over a hundred miles between RF transmitter and receiver. RF frequency range is 30 KHz to 530 KHz and output power is typically 1, 10 or 100 watts.
The term “Narrow Band” typically means receiver bandwidth of 300 Hz or 600 Hz or 1200 Hz. On/Off keying means simply turning the RF carrier on or off at command of the associated protective relay. For frequency shift keying (“FSK”), the transmitter is on all the time but shifts in frequency as commanded by the associated relay. There are usually two or three frequencies to select from, with the frequencies being mutually exclusive. All frequencies for a particular FSK communications link are within the receiver's bandwidth.
The transmitter section typically has a self contained RF output power monitor to continuously measure forward power and reflected power. There will be no reflected power if the transmission line load impedance exactly matches the transmitter output impedance. Full output power cannot be delivered to the load if there is a mismatch. A large mismatch indicated by high percent reflected power implies that RF coupling equipment between transmitter and AC power line has a serious defect that needs repair.
A problem with the conventional type of forward/reflected power monitor is that it has very wide bandwidth and may be affected by RF signals way out of the narrow band of interest that the transmitter occupies. Said interfering signals are applied from the RF transmission line back to the transmitter and may be caused by, among other things, other PLC transmitters that share the power line, and even high power broadcast stations being picked up by the long power line acting as an antenna. The primary effect of this interference is to cause a reflected power measurement error.
One known solution to this problem for providing power monitoring for a power line carrier is to use a dedicated second receiver with a filter for the power meter. However, this second receiver and filter solution is far from cost effective. Another, alternative, solution is to re-program the main, single receiver to the transmit channel frequency to indicate reflected power. However, when this is done, the receiver can no longer do its primary task of protective relaying and, therefore, the reprogramming of a single receiver for power measurement can only be done when the power line carrier equipment is taken out of service.